Such a differential gear is commonly used in motor vehicles to distribute the drive power from the drive engine to two driving wheels. The differential gear housing carries a crown wheel meshing with a differential drive pinion which in turn is driven by the motor. The side wheels are directly connected to a respective output shaft carrying a driving wheel. Known differential gears have the serious drawback that they function as intended only when the driving wheels are in full frictional engagement with the ground. As soon as a driving wheel looses its full engagement, the driving capacity of the other driving wheel is correspondingly decreased so as to entirely cease when the first driving wheel has lost all frictional contact with the ground. This peculiarity of conventional differential gears involves great risks in skid situations, particularly in connection with the so called secondary skid. A secondary skid occurs after a primary skid when both driving wheels, according to the above, have lost their driving capacity. During the primary skid the driving wheel leading in the direction of skidding has at least some engagement in the direction of traction, while the other driving wheel is more or less raised from the ground. The differential gear then transfers substantially all drive power to the easiest driveable driving wheel, i.e., the raised one, in which is built up a large mass energy.
When the primary skid ceases, this driving wheel regains its contact with the ground, whereupon the stored energy is instantaneously transferred to the ground, resulting in a violent thrown in the opposite direction. Unlike the primary skid, which is built up relatively slowly, the secondary skid occurs considerably faster and often has serious consequences.
It is apparent from the above presentation that there is a need to be able to control the power distribution to the output shafts of a differential gear. A known extremity in this respect is the differential lock that blocks the distributing function of the differential gear such that the output shafts are rigidly interconnected. The advantages and disadvantages of such lock herewith are well known. Further, so called anti-spin systems are known which lower the drive power until wheel spin ceases, but which cannot direct the drive power to a selective driving wheel.